If you are only wanting to override a few font colours for the site then you can add the relevant css information to:

htdocs/local/theme/style/style.css

If the file/path does not exist create it first.

This style sheet gets called last for the page and so can override the particular style for all themes, eg

body {
color: #ff0000;
}

would make all text (not getting a specific style) be red-----

However, if you are wanting to develop your own theme where you expect to make may changes to the default theme you can use 'Deafult' as a base and copy the htdocs/theme/default theme directory to a new name, eg

cp -r htdocs/theme/default/ htdocs/theme/mytheme/

Then go into htdocs/theme/mytheme directory and edit the themeconfig.php file and change

$theme->displayname = 'Default';

to

$theme->displayname = 'MyTheme';

and then edit htdocs/theme/mytheme/sass/utilities/_theme-variables.scss

and change the color information there to what you want

then in the parent dir for htdocs and run:

make css

This will generate the css files for your new theme.Then you'll need to choose this new theme in the admin sectionto see it for your site.